Many attempts have been made to measure the cylinder pressure directly in the cylinder, using piezoelectric and strain gauge sensors. Due to carbonization and corrosion influences, however, it has not been possible to design a dependable sensor capable of functioning inside the combustion chamber and with the necessary accuracy for the required operating period of 4000 to 8000 hours. Consequently, monitoring systems have been devised with pressure sensors outside the combustion chamber, typically on the indicator connection still provided on every diesel engine today. The sensor is then connected to the cylinder from time to time by a manually or automatically operated valve. This intermittent monitoring system is unable to detect phenomena occurring suddenly and briefly, which may be important under certain circumstances. Consequently it has not been adopted widely.
Indirectly measuring cylinder pressure measuring systems have appeared recently, based on the strain of cylinder studs or cylinder force conducting parts. Piezoelectric load washers under the nuts of cylinder cover studs were employed as long as 20 years ago for research purposes and have yielded cylinder pressure curves of very high information value. However, such washers were too high and too costly for a monitoring system. On account of their height, they would require special fitting procedures. The same is true of measuring washers fitted with strain gauge elements, which are also not suitable for continuous operation.
The object of the invention is to create an informative monitoring system for engines, compressors and other cyclically operating machines, which is suitable for continuous operation and easy to install and remove.
The object is achieved by sensors of the system detecting both the force versus pressure curve in each cylinder and the structure-borne noise signals, and comparing the detected values with prestored values in a computer, to analyze the engine state. The present state can thus be compared continuously with the state when new.
A principal feature of the invention is the use of the equivalent variable cylinder force in place of the cylinder pressure. This force provides a pattern fully correlated to the cylinder pressure throughout the running time of the plant or engine, regardless of local carbonization. By recording the nominal force curves for all cylinders under a number of load states, in new condition and after plant overhaul, and storing these curves in a computer, an elegant and reliable comparison possibility is obtained between the particular actual state and the nominal state, without having to know the absolute magnitude of the cylinder pressure. This provides a monitoring method that is not rendered uncertain by erroneous indication procedures and functions irreproachably over the required running time of 4000 to 8000 service hours.
According to the invention, a selected cylinder cover stud of each cylinder cover is equipped with a thin disk sensor, which replaces the standard washer of the same overall height. The nut is then tightened with normal torque. Accordingly, fitting these monitoring sensors is very simple and requires no reworking.
The elasticity behavior of the screw connection is unchanged, because the thin disk sensor consists mainly of steel.
It is important that a cylinder cover stud as far away as possible from the neighboring cylinders should be equipped with a sensor, in order to minimize interferences.
The cylinder force measurement is related directly with the pressure measurement in the cylinder by comparative measurement of the cylinder pressure measured on the indicator connection. For this, the signal measured in the pressure sensor is compared from time to time, especially after any engine overhaul, by a simple calibration procedure with the cylinder pressure.
For the monitoring function, however, the cylinder force diagram is quite sufficient as relative pressure diagram, because it reproduces all details of the cylinder pressure diagram.
Only the relative change in time of the engine state during the service period of 4000 to 8000 hours is of importance for monitoring. Also important is that every cylinder should be monitored.
Equally important is a TDC mark of reliable accuracy, fed into the data acquisition system. It is advantageous to make this mark on the flywheel, related to the cylinder nearest to this. It is evaluated for this cylinder only.